1. Field of the Invention
The present invention generally relates to a power supply control device for driving a plurality of switching power supply circuits which are incorporated in a power supply unit and have different properties, for example, the ordinary rectification type switching power supply circuits and the synchronous rectification type switching power supply circuit by synchronizing the operation timing among the circuits.
2. Description of the Related Art
In recent years, electronic devices have been miniaturized, while the performance thereof and the range of functions thereof have been enhanced. Thus, a power supply unit to be incorporated in an electronic device has a plurality of switching power supply circuits respectively corresponding to loads (namely, machines and circuits) of different properties. Here, note that generally, the pulse width modulation type control integrated circuits are used for controlling the driving of the switching power supply circuits and that a power supply unit has a number of control systems which corresponds to the number of the loads, namely, the number of the switching power supply circuits.
Meanwhile, when operating a plurality of switching power supply circuits in a same power supply unit independent of one another, an interference phenomenon (so called a switching beat), which is based on a plurality of switching operations, sometimes occurs in the power supply unit. This is dye to the fact that, for example, when two switching power supply circuits operate at switching frequencies f1 and f2, respectively, a low frequency switching beat corresponding to the difference (f1-f2) between the two switching frequencies results.
In order to prevent an occurrence of this low-frequency switching beat phenomenon, the conventional power supply unit synchronizes the timing of switching operations among the plurality of switching power supply circuits that undergo independent output control. As practical examples of this, there have been employed the following two means.
(1) Namely, a first means adapted to employ a multi-channel type control device which uses a unified reference waveform signal (namely, a triangular-wave voltage) as those of a plurality of control systems constituted in the control device. PA0 (2) Further, a second means adapted to employ control devices, each having a master-slave function, and to use one of the control devices as a master control device and operate this control device at a predetermined operation timing and at a predetermined oscillation frequency, and to use the rest of the control devices as slave control devices which causes the slave control devices to receive a synchronization signal from the master control device and operate at an operation timing synchronized with the master control device and at the same oscillation frequency as that of the master control device.
FIG. 1 schematically illustrates the control device corresponding to the first means, which is incorporated with the intention of preventing an occurrence of a low-frequency switching beat phenomenon, and a power supply unit employing this control device.
The control device CD of FIG. 1 is roughly composed of a first control system CH1, a second control system CH2, a reference voltage supply 1, an oscillation circuit 2 and a protective circuit 3.
Here, the reference voltage supply 1 is adapted to output a highly-stable voltage as a reference voltage level. Further, the oscillation circuit 2 is operative to output a triangular-wave voltage at a predetermined oscillation frequency. Moreover, the protective circuit 3 is operative to output an operation stopping signal so as to protect the systems from a short in the output circuit and to prevent an occurrence of a malfunction when a low level signal is input.
The reference voltage supply 1, the oscillation circuit 2 and the protective circuit 3 are shared by the control systems.
The control system CH1 is used for driving and controlling a first power supply circuit PS1 and is basically composed of an error amplifier Err1, a comparator Comp1 and an output buffer 1. An operation of this control system CH1 is performed as follows. Namely, first, this control system CH1 receives a feedback signal FB1 from the switching power supply circuit PS1. Then, a voltage level represented by the received feedback signal FB1 is compared with a reference voltage (V.sub.REF) in the error amplifier Err1, so that an error signal is obtained therein. Subsequently, this error signal is compared with a triangular-wave voltage in the comparator Comp1, which generates a pulse output signal PO1 having on-duty in accordance with an output voltage thereof. This pulse output signal PO1 is supplied to a switching device of the switching power supply circuit PS1. Thereby, the switching power supply circuit PS1 is driven and controlled in such a manner that an output voltage thereof remains constant.
The control system CH2 is used for driving and controlling a second power supply circuit PS2 and is basically composed of an error amplifier Err2, a comparator Comp2 and an output buffer 2. An operation of this control system CH2 is similar to the operation of the control system CH1. Thus, the description of the operation of the control system CH2 is omitted.
Here, the protective circuit 3 is adapted to output an operation stopping signal when an abnormality occurs in the switching power supply circuits PS1, PS2. When receiving this operation stopping signal, the output buffers Buf1, Buf2 prohibit pulse output signals PO1, PO2 from passing therethrough. Thus, an operation of the switching power supply circuit PS1 is stopped.
Referring next to FIG. 2, there is schematically shown control devices corresponding to the second means and a power supply unit having these control devices, which are constituted for the purpose of preventing an occurrence of low-frequency switching beat phenomenon.
In the power supply unit of FIG. 2, two switching power supply circuit PS3 and PS4 are driven and controlled by control devices MCD and SCD, respectively, independent of each other.
Here, the control device MCD is composed of: a control system, which consists of an error amplifier MErr, a comparator MComp and an output buffer MBuf; a reference voltage supply 11 for outputting a reference voltage or signal; an oscillation circuit 12; and a protective circuit 13. An operation of this control device MCD is performed as follows. Namely, the control device MCD receives a feedback signal FB3 from the switching power supply circuit PS3. Subsequently, a voltage represented by this feedback signal FB3 is compared with a reference voltage (V.sub.REF) in the error amplifier MErr, so that an error signal is obtained.
Then, the comparator MComp compares the error signal with a triangular-wave signal sent from the oscillation circuit 12 and further outputs a pulse output signal PO3 having on-duty in accordance with an output voltage thereof. This pulse output signal PO3 is supplied to a switching device of the switching power supply circuit PS3. Thereby, the switching power supply circuit PS3 is driven and controlled in such a manner that an output voltage thereof remains constant.
Here, the oscillation circuit 12 is operative to output a synchronization signal RSS (which is generally a voltage that is the same as the triangular-wave voltage to be supplied to the comparator Comp1) to an external circuit simultaneously with supplying a triangular-wave voltage to the comparator MComp.
Further, the control unit SCD is composed of: a control system which consists of an error amplifier SErr, a comparator SComp and an output buffer SBuf; a reference voltage supply 21; an oscillation circuit 22; and a protective circuit 23. An operation of this control device SCD is fundamentally the same as of the control device MCD. The control device SCD, however, is different from the control device MCD, in which the oscillation circuit 12 of the control device MCD produces a triangular-wave voltage at independent operation timing and at the predetermined oscillation frequency, in that the oscillation circuit 22 of the control device SCD receives a synchronization signal (RSS) from the control device MCD and operates in such a way as to generate a triangular-wave voltage whose operation timing and oscillation frequency are the same as those of the signal produced by the oscillation circuit 12.
As is understood from this, in the case of the power supply unit of FIG. 2, the control device MCD functions as the master control device, while the control device SCD functions as the slave control device.
The reduction in size of electronic devices in recent years has resulted in great demand for the miniaturization of power supply units. When miniaturizing the power supply units, the inductors and the capacitors of the switching power supply circuits PS3 and PS4 are first miniaturized. At that time, the switching frequency of the switching device is set at a high value. This switching frequency is determined according to the oscillation frequency of a triangular-wave voltage output by the oscillation circuit 2, 12 or 22 of the control device CD, MCD or SCD. Although the generally used oscillation frequency is several hundreds kilohertz (kHz), an oscillation circuit, which provides an oscillation frequency of several megahertz (MHz), is currently available.
Meanwhile, because of the fact that batteries are used in electronic devices as a power source, the reduction in size of the electronic devices to the portable size thereof also results in great demand for a reduction in the power consumption of the power supply units. A problem to be solved by the reduction in power consumption of the power supply units to low levels is the improvement of the efficiency of each of the switching power supply circuits. Switching power supply units of the synchronous rectification type are available as means for improving the efficiencies of the switching power supply circuits.
This switching power supply circuit of the synchronous rectification type employs a transistor as a rectifying device, instead of a diode. A loss produced in the transistor due to a collector-to-emitter voltage in an on-state is less than a loss produced in the diode owing to a forward voltage (drop). Thus, theoretically, a loss produced in the rectifying device is reduced. Thereby, the efficiency of the switching power supply circuit can be enhanced. However, in the case of actually using a transistor as a rectifying device, losses are produced when on/off operations thereof are performed. As the switching frequency becomes higher, losses abruptly increase. As a result, in the case of switching frequencies of a certain range, a loss produced in the switching power supply circuit of the synchronous rectifying type using a transistor is more than a loss produced in the switching power supply circuit of the ordinary type using a diode. Incidentally, it is known that in the case of using semiconductor devices already on the market, the limit of the switching frequency for increasing the efficiency of the switching power supply circuit by employing the circuit of the synchronous rectification type is about 200 kHz.
Moreover, in the case of the circuit of the synchronous rectifying type using a transistor as the rectifying device, there is the necessity of complementarily driving the transistor, which is employed as the rectifying device, in synchronization with the operation of the switching device. Thus, the switching power supply circuits further require circuits and components therefor.
Therefore, in the case of the switching power supply circuit employing the synchronous rectification, the size of the circuit is large in compensation for the increasing of the efficiency thereof, in comparison with the case of employing the switching power supply of the ordinary rectification type. Consequently, in the former case, the size of the power supply unit is large.
To attain the miniaturization of the power supply unit and the increasing of the efficiency thereof by achieving a good balance therebetween, it is considered as necessary to suitably arrange the switching power supply circuit of the ordinary rectification type and the switching power supply circuit of the synchronous rectification type in the unit according to the characteristics of loads. For example, the miniaturization of the power supply unit is achieved by applying the switching power supply circuit of the ordinary rectification type to a load whose power consumption is small, while the high efficiency of the power supply unit is realized by applying the switching power supply circuit of the synchronous rectification type to a load whose power consumption is large.
However, in the actual case of using the switching-power supply circuit of the ordinary rectification type together with the switching power supply circuit of the synchronous rectification type, the conventional power supply control device of FIG. 1 has a problem in that pulse output signals supplied from each of the control systems to the switching power supply circuits has only one kind of a pulse frequency (a switching frequency).
The miniaturization of the power supply unit requires a high switching frequency, whereas the increasing of the efficiency thereof (synchronous rectification) requires a lower switching frequency. Namely, when the pulse output signals has only one kind of the pulse frequency under this condition in which the tendencies of the frequencies respectively required by the miniaturization and the increasing of the efficiency, respectively, are opposed to each other, one of the miniaturization and the increasing of the efficiency has to be traded off for the other thereof.
Similarly, in the case of the power supply control devices (MCD and SCD) of the master-slave type as shown in FIG. 2, in order to prevent generation of a low frequency switching beat, the operation-timing and oscillation-frequency of a triangular-wave voltage outputted from the slave control device (SCD) are set in such a manner as to be the same as those of a triangular-wave voltage outputted from the master control device (MCD). Thus, the same problems as in the case of the power supply control circuit of FIG. 1 comes up. Consequently, one of the miniaturization and the increasing of the efficiency has to be traded off for the other thereof.